This invention is in the field of photovoltaic cells and in particular in the field of multi-junction, multi-element photovoltaic cells.
Photons of incident solar radiation strike the photovoltaic cells of a solar panel and are absorbed by semiconducting materials, such as silicon. A common type of photovoltaic cell is a single layer silicon cell made from a semiconductor wafer. The photovoltaic material of a photovoltaic cell is commonly formed by doping a thin layer at the top and bottom of the silicon layer of the cell, producing a pn-junction with a particular bandgap energy, Eg. The silicon layer has metallic contacts on the top and bottom. Photons of the incident solar radiation may be reflected or transmitted into the cell. Each of the transmitted photons gives up its energy to an electron, if the energy of the photon is equal to or greater than the bandgap energy, resulting in electrons being excited from their atomic or molecular orbits by the photons, generating electron-hole pairs. Excited electrons and holes respectively travel towards n-doped and p-doped regions at the top and bottom of the layer and to electrodes of the cell, resulting in a generation photocurrent Ig. Voltage across the cell is generated from the incident solar radiation which results in the Ig, which is a direct current (DC), and the energy of the direct current can be stored in batteries, capacitors or other energy storage processes. The direct current may also be converted to alternating current and discharged to a grid.
The maximum theoretical efficiency of traditional single-junction cells is 34%. This is known as the Shockley-Queisser limit. This limit is based on several loss mechanisms that are inherent to any solar cell. One type of loss is the blackbody radiation loss which accounts for about 7% of the power loss. A second type of loss is known as “recombination”, which accounts for about 10% of the power loss, and results from the electrons created by the photoelectric effect being captured in the electron holes left behind by previous photon-electron excitations. A third and principal loss mechanism results from bandgap limitations, i.e. result from the fact that a photon must have enough energy to exceed the bandgap energy of the material, or it will not excite an electron from its orbit. Bandgap limitations are a major problem for single layer, single junction solar cells. For instance, depending on the semiconductor material used for the cell, the cell may not be responsive to a large portion of the infrared spectrum, which represents almost half of the energy of incident solar radiation. Also, a fourth type of loss, referred to as “relaxation,” may occur when photons with a higher energy level than the bandgap excite electrons well above the bandgap and the extra energy is lost through electron collisions. This relaxation energy loss is in the form of heat in the cell which may further increase blackbody losses.
All of the losses total at least 66% of the energy of the incident solar radiation. Other design concerns, such as reflection off the front surface or the metal terminals, further reduce efficiency. The best presently commercially available single layer/single junction cells have an efficiency of about 25%.
Multi-junction solar cells are solar cells of multiple layers of differing semiconductor materials with multiple p-n junctions. The p-n junction of each layer will produce electric current in response to a different bandwidth of radiation, resulting in the cell producing electric current in response to multiple bandwidths of radiation. Although as discussed above, a single layer cell has a 34% efficiency limit, multiple layer/junction cells have a theoretical efficiency limit of approximately 87% corresponding to an infinite number of layers/junctions. The higher the number of layers, the higher the theoretical efficiency limit of the cell. However, there is an exponential relationship between theoretical efficiency and the number of layers/junctions required. While multi-junction/layer cells with efficiencies exceeding 40% are commercially available, the cost/performance ratio of single layer cells has been generally better than the multi-junction cells presently available.
Multi-junction cells typically have a layer with the shortest wavelength bandgap as the top layer, with the wavelength bandgap of the subsequent layers progressively increasing from the top layer to the bottom layer. Transparent conductors are required to provide for unabsorbed radiation passing each layer to be transmitted to the next layer. Multi-junction cells are difficult to produce because of the thinness of the materials and the difficulties with extracting the current between the layers. Amorphous silicon cells, which are mechanically separate but electrically connected address this issue.
A monolithically integrated cell having multiple layers that are mechanically and electrically connected, is difficult to produce. Since the layers are connected in series, the same current must flow through each junction, and if the photocurrents generated in each layer are not matched, electrons will be absorbed between layers. Bandgaps of each layer must be chosen to balance the current generation in each layer.
For a typical three layer, multi-junction photovoltaic cell, the top layer may be designed to absorb a portion of the ultraviolet spectrum and perhaps a portion of the visible light spectrum, and to pass the remaining wave lengths of visible light and infrared. A second layer may be designed to absorb the visible light passed by the top layer and to pass infrared. A third layer may be designed to absorb as much of the shorter wavelength infrared spectrum as practical. Each of the three layers must be comprised of material specifically selected and must be designed to produce as close as possible the same photocurrent as the other two layers for the radiation absorbed in its layer bandgap.
Solar concentrators, such as Fresnel lenses and parabolic reflective lenses can be used to increase the cost/efficiency ratio of a photovoltaic solar collector. The multiplier on the solar radiation concentration provided by the concentrator lens, particularly if the lens is equipped with solar tracking, may offset the cost/efficiency disadvantage of the multi-junction photovoltaic cells. Accordingly, much of multi-junction photovoltaic cell research involves use with solar concentrators.
The difficulty in matching the photocurrent produced by each layer of a multi-layer photovoltaic cell is further complicated by the variations in the power distribution over the overall operating spectrum range, the “overall bandgap,” for which energy is intended to be absorbed by the photovoltaic cell. Significant variations in the power distribution among the ultraviolet, visible light, and infrared spectrums occur with variations in the time of day, season, latitude, altitude, and cloud cover. Even variations in atmospheric pressure and humidity may significantly affect the power distribution over the overall bandgap. Photovoltaic layer material selection and layer design for a sea level, equatorial, frequent cloud cover, and high humidity application, may be poorly suited for a high latitude, high altitude, clear sky, and low humidity application. Further, a photovoltaic cell with material and layer design selected to optimize efficiency during a particular season, may result in a substantially reduced efficiency during other seasons, when the power distribution within the overall bandgap will be substantially different. Still further, a photovoltaic cell which has its layer material and layer design selections made based upon a particular time of day, i.e. optimized based upon the power distribution within the overall bandgap during a particular time of day, for example solar noon, may result in substantially diminished efficiency during other times of day, particularly the early morning and later afternoon hours. In each operating condition, the photocurrent produced by the least productive photovoltaic layer will determine the cell output photocurrent, and hence the power output and efficiency of the photovoltaic cell.
One solution to this problem of photocurrent differential between layers is to physically and electrically isolate the photovoltaic layers and to combine the current from each layer outside the photovoltaic cell. This is referred to as an amorphous photovoltaic cell.
Another inherent problem affecting the efficiency of a photovoltaic cell, or a photovoltaic layer of a multi-layer cell is related to the output voltage range of the layer. Referring to FIG. 4, the instantaneous current I 71 generated by each photovoltaic cell layer and flowing from the photovoltaic cell layer will be dependent upon the extent of the irradiation of the photovoltaic cell layer, the characteristics of the photovoltaic cell layer, and the instantaneous voltage V 73 of the circuit to which the current is being discharged by the photovoltaic cell layer. For a multi-layer cell, the foregoing is true of each layer.
The formula for the instantaneous power (P) generated by the photovoltaic cell may be determined by the formula P=I*V. The I and V values at which the maximum power Pmax 79 is generated are I=Imp 81 and V=Vmp 77 respectively. I is at its maximum I0v 78 when the voltage in the circuit to which the photovoltaic cell layer is being discharged is zero. In the case of a photovoltaic cell layer producing charge that is stored in a capacitor, the maximum current occurs when there is no charge on the capacitor. As the V to which the photovoltaic cell layer is subjected by the capacitor charging circuit, increases above Vmp, the current produced by the photovoltaic cell layer decreases rapidly and goes to zero at the voltage reaches the shut-off voltage (Vs). During any time period that V in the circuit to which the photovoltaic cell layer or the photovoltaic array as a whole, is discharging current, exceeds Vmp, the efficiency of the photovoltaic cell layer or photovoltaic array as a whole will be significantly diminished.
Energy from a photovoltaic system is generally stored in batteries for later use or converted to an AC current for discharge to an electrical grid. If the energy is to be stored in a battery, the voltage for the photovoltaic system will have to be adjusted to exceed the transient voltage of the battery. Since the maximum voltage output of a photovoltaic cell or a photovoltaic cell layer of a multi-layer photovoltaic cell, is typically on the order of 0.5 volts, the voltage must be stepped up before the energy can be stored in a battery system. Similarly, if the energy generated by a photovoltaic cell is to be discharged to an electrical grid system, which may be operated at 240 volts, 480 volts, or much higher voltages, the voltages must be stepped up to a voltage exceeding the minimum voltage required by an inverter which will invert the DC current to a pulsed AC current. Various filters may be used to impose a sinusoidal wave form on the AC.
To maximize the efficiency of a multi-layer photovoltaic cell, the problem of mismatched photocurrent of the photovoltaic layers of a monolithically integrated photovoltaic cell, and the problem of the photocurrent being reduced as the discharge voltage increases on the photovoltaic cell, must be addressed.
For purposes of this application, including but not limited to, the Summary of the Invention, the Brief Description of the Drawings, the Detailed Description, the Claims, and the Abstract, the term “photovoltaic layer” shall be defined to include a layer of material having the characteristic and ability to receive and absorb electromagnetic radiation and to generate a current, namely a photocurrent, through the absorption of the electromagnetic radiation; the term “photovoltaic element” shall be defined to include a photovoltaic layer and one or more other functional layers or components, such as window layers, anti-reflective coatings, conduction layers, or metallic contacts; and the term “multi-element photovoltaic cell” shall be defined to include a photovoltaic cell having two or more photovoltaic elements. The term “electromagnetic radiation” includes particularly the ultraviolet, visible light and infrared spectrums respectively.
An objective of the present invention is to provide a multi-element photovoltaic cell having a photovoltaic controller, which provides for the continuous production of current by each of the irradiated photovoltaic cells of a photovoltaic array regardless of the level of irradiation.
A further objective of the device and method of the present invention is to provide for the continuous production of current by a photovoltaic cell by avoiding increasing the voltage of the discharge circuit, at each of the photovoltaic cells, above Vmp.
A further objective of the present invention is to provide for the continuous and optimized production of energy by each of the photovoltaic elements of a photovoltaic cell assembly while simultaneously stepping up the voltage of an aggregate current discharged by the photovoltaic cell assembly to a level required for discharge to an inverter or to a DC battery storage system, or both.
A further objective of the present invention is to provide for the continuous and optimized production of energy by each of the photovoltaic cells of a photovoltaic array while simultaneously stepping up the voltage of an aggregate current discharged by the full photovoltaic array to a level required for discharge to an inverter or to a DC battery storage system, or both.